Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic signals (sound waves) and convert received acoustic waves to electrical signals via inverse and direct piezoelectric effect. Acoustic transducers generally include acoustic resonators, such as thin film bulk acoustic resonators (FBARs), solidly mounted resonators (SMRs), surface acoustic wave (SAW) resonators or bulk acoustic wave (BAW) resonators, and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. For example, FBARs may be used for electrical filters and voltage transformers. Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may form a thin membrane. FBAR and SMR devices, in particular, generate longitudinal acoustic waves and lateral (or transverse) acoustic waves when stimulated by an applied time-varying electric field, as well as higher order harmonic mixing products. The lateral modes and the higher order harmonic mixing products may have a deleterious impact on functionality.
In certain configurations, a frame may be provided along one or more sides of an FBAR or an SMR to mitigate acoustic losses at the boundaries by improving the confinement of electrically excited modes in the active region of the FBAR (the region of overlap of the top electrode, the piezoelectric layer, and the bottom electrode). In general, frames create an acoustic impedance mismatch that reduces losses by suppressing excitation and thus minimizing scattering of FBAR or SMR modes at the edges of the electrodes (predominantly the top electrode). Also, frames reflect electrically excited propagating modes back to the active area of the resonator, and therefore improve confinement of these modes within the active region of the FBAR or SMR. Frames located along the sides of FBARs or SMRs generally increase parallel resistance (Rp). A typical frame provides two interfaces (impedance miss-match planes), which increase reflection of propagating eigenmodes in lateral directions. When the width of the frame is properly designed for a given eigenmode, it results in resonantly enhanced reflection and suppression of that particular eigenmode, thus yielding better energy confinement and higher Q-factor at parallel resonant frequency (Fp).
However, better acoustic energy confinement, as well as further improvements in FBAR or SMR Q-factor due to the better acoustic energy confinement, are needed for increased efficiency of FBAR or SMR devices.